Minimizing sulfur oxidation in pulping liquor reconstitution

ABSTRACT

Disclosed is an improved process for the liquid phase coking of spent wood pulping liquors wherein a part of the decoked effluent is flash vaporized to supply condensate for SO2 absorption, thus eliminating the need to absorb SO2 from an oxygen-containing flue gas into sulfite ion-containing coker effluent, thereby minimizing sulfite ion oxidation and reducing the amount of makeup chemicals required to reconstitute the pulping liquor. The process is applicable to all acid sulfite pulping processes employing metallic ions such as sodium, calcium, magnesium, to ammonia-base sulfite and alkaline sulfite pulping liquors.

United States Patent 1191 Franz et al. 1

"1111 3,816,240 June 11, 1974 MlNlMlZlNG SULFUR OXIDATION lN 3,644,087 2/1972 Urban 423/242 PULPING LIQUOR RECONSTITUTION 3,649,534 3/ 1972 Schotte I 3,717,545 2/1973 Hess et a1. 162/31 [75] Inventors: William F. Franz, Gardiner; Howard f Glenham; Edward Cole, Primary Examiner-Robert L. Lindsay, Jr. Flshkllh of Assistant Examiner-Alfred DAndrea, Jr. [73] Assign/3c; Texaco Inc. New York NY. Attorney, Agent, or Firm-T. H. Whaley; C. G. Reis 1 1 pp Nod 2651099 Disclosed is an improved process for the liquid phase coking of spent wood pulping liquors wherein a part of 52 us. c1 162/31 162/36 162/47 decoked effluent is flash vaporized Supply ,123/207,423/242 densate for S0 absorption, thus eliminating the need 511 int. Cl... D216 11/02, D216 1 1/06, D2 161 1/14 absofb 502 9"? OXYEeH-COMainiHE flue 5 5 58 Field of Search 162/30 31 36 47- Sulfile effluent thereby mlmmiz 2l0/63. 423/182 6 ing sulfite ion oxidation and reducing the amount of make-up chemicals required to reconstitute the pulp- [56] References Cited mg The process 1s applicable to all acid sulfite pulping UNITED STATES PATENTS processes employing metallic ions such as sodium, 2,752.243 6/1956 Barton et a1. 162/31 calcium, magnesium, to ammonia base lflt and 33639 Z5133? Siififif..i.'j.ii ""jjjii'z'zs/ z'fz i alkaline sulfite pu'ping l 3.595.806 7/l97l Prahacs et a1. 162/30 X 4 Claims, 1 Drawing Figure FEED y ,21 FLASHER 12B '5 ABSORBER FLASH DRUM uil 'fifii HOLD 3441 TO 25 1 -3 meEsToRs' wii iiER g???- 24 v29 1 MINIMIZING SULFUR OXIDATION IN PULPING LIQUOR RECONSTITUTION FIELD OF THEINVENTION This invention relates to an improved process for treating spent-wood pulping liquors which results in the reconstitution of the wood pulping liquors with consequent savings in chemicals and in the substantial elimination of disposal and pollution problems.

DESCRIPTION OF THE PRIOR ART A portion of the sulfur present in the spent liquor remains in solution in the coker liquid effluent, largely as the desired sulfite ion, provided care is taken to exclude air in the processing steps. So readily does oxidation of sulfite to sulfate take place that sodium sulfite has long been used as an oxygen scavenger in boiler feed water treatment.

The remainder of the sulfur is present on the coke and is only incompletely removed in the washing step for sodium recovery. This bound sulfur on the coke must be recovered in the form of SO; from the flue gas when the coke is burned. Efficient combustion of the coke requires that excess combustion air be used, and consequently the flue gas may'contain up to several percent of free oxygen. Absorption of S from this oxygen-containing flue gas by the coker liquid effluent would result in oxidation to sulfate of a portion of the sulfite dissolved in it. Since the sulfate form is inactive in subsequent pulping operations, any sulfite oxidized to sulfate represents a loss of sulfur to the process. Consequently, it is of benefit to avoid contact between the oxygen-containing flue gas and the sulfite-containing coker liquid effluent.

SUMMARY OF THE INVENTION Absorption of S0 from the flue gas in condensate flashed from the main coker liquid stream avoids the undesirable contact of oxygen with the sulfite ion while accomplishing the desired purpose of recovering the $0 in the flue gas.

DETAILED DESCRIPTION OF THE INVENTION In the physical embodiment of the invention diagrammed in the drawing, spent pulping liquor is raised to coker operating pressure of LOGO-1,100 psig by pump and raised to coker operating temperature, 450 to 700F, in the absence of added free oxygen under autogenous pressures of about 1,000 to 3,000 psig in fired heater 11. The partly coked liquor then passes to coker hold drum 12 for a period of l to 60 minutes, sufficient to complete the coking reactions and to allow the coke formed in these reactions to settle to the bottom of the hold drum. Gases formed in the course of the coking reactions are removed overhead through line 14. Clear coker liquid effluent is removed from the hold drum through line 15 and reduced in pressure by passage through throttling valve 16. Actual pressure reduction through valve 16 will be determined by the liquid rate requirement of SO, absorber 23, but generally will'be reduced to around psig or less.

Flash drum 17 serves to separate remaining liquid phase material from the flashed vapor, which passes overhead to condenser 18. Gaseous products of the coking reactions pass through line 19 to join gases from coker hold drum 12 at point 20. Consequently, the

combined gas stream then passes through line 21, to be burned in heater 11 to destroy malodorous sulfur compounds. Condensate from 18 passes through line 22 to SO; absorber 23.

Settled coke passes from the bottom of hold drum 12 to coke dewatering device 24. Liquid separated from the coke rejoins the main coker liquid stream through line 25 while a heavy coke slurry passes through line 26 to coke washer 27 for removal of soluble inorganics. Washed coke passes to a second coke dewatering device 28 and then to coke dryer 29. Here residual water is evaporated from the coke by hot flue gas from heater ll. Dried coke passes through line 30, to be burned as fuel in heater ll.

Flue gas exits from coke dryer 29 through line 31 and passes to S0 absorber 23. Here S0, is removed by absorption in condensate from line 22. Flue gas, free of S0 is vented through line 32 while SO, solution passes through line 33 to coke washer 27. Effluent from 27, containing S0 recovered from flue gas and sodium washed from the coke, rejoins unvaporized liquid from flash drum 17 at point 34. The recombined liquid stream, after addition of any make-up chemicals which might be required, is then returned to the pulping digesters.

The practice of the invention leads to a saving of from 20 to 100 percent in make-up chemicals.

To avoid gel-type coke formation where ammoniabase sulfite liquor is being handled in the present process, it is preferred to heat the spent pulping liquor rap idly'at a rate of 1 10 to l50F per minute in the temperature interval of about 350F to about 550F. This preferred feature is disclosed and claimed in our copending application Ser. No. 149,487, filed June 3, 1971.

To increase the rate of coking and to carry out such coking at a lower temperature, the pH of waste alkaline liquors may be lowered to the range of about 7 to about 3 by adding sulfur dioxide to the waste liquor either as free S0 or as an aqueous solution of S0 This preferred feature is disclosed and claimed in our copending application Ser. No. 149,425, filed June 3, l97l.

What we claim is: r

1. In a process for liquid phase coking of spent sulfite-based pulping liquors to produce coke, flue gases including sulfur dioxide, and an aqueous sulfitecontaining decoked effluent: the steps of: flash vaporizing a portion of said effluent; condensing said portion to form a condensate; immediately contacting said condensate with hot oxygen-containing flue gas containing sulfur dioxide formed by the drying and burning of said coke in excess combination air to form a sulfite solution thereby avoiding contact between said flue gas and said sulfite-containing effluent; and then washing said coke with said sulfite solution prior to drying and buming to remove soluble inorganic salts therefrom whereby chemicals are recovered for reconstitution of wood pulping liquors, and combining the said solution which now contains said salts with the unvaporized portion of said effluent.

2. The process of claim 1, wherein said flash vaporizing is effected by flowing said decoked effluent to a flashing zone and reducing pressure thereon to about 100 psig.

3. The process of claim 1, wherein the pH of said Ii- 5 550F.

quors is reduced to between about 7 and 3 before cok- 

2. The process of claim 1, wherein said flash vaporizing is effected by flowing said decoked effluent to a flashing zone and reducing pressure thereon to about 100 psig.
 3. The process of claim 1, wherein the pH of said liquors is reduced to between about 7 and 3 before coking.
 4. The process of claim 1, wherein said liquors are coked by heating at a rate of 110* to 150*F per minute in the temperature range of about 350*F to about 550*F. 